1. Field of the Invention
The present invention relates to a compound semiconductor device having a transistor configuration which includes a buffer layer containing GaN and the fabricating method of the same.
2. Description of the Related Art
Amplifiers for the base-stations for recent portable phones are required to operate at high voltages and thus they must exhibit excellent withstand voltages. As electronic devices for achieving excellent withstand voltages, there have been actively developed compound semiconductor devices having a HEMT-configuration employing sapphire, SiC, GaN, Si or the like as a substrate, GaN as an electron transit layer and AlGaN as an electron supply layer (hereinafter, referred to as GaN-HEMTs). For example, Japanese Patent Application Laid-open No. 2000-106365 discloses a technique in which an AlN layer or an AlGaN layer is formed under a gate electrode to improve the interface state. GaN has a band gap of 3.4 eV, which is significantly larger than that of GaAs of 1.4 eV. A GaN-HEMT can create an amount of 2-dimension electron gas which is about ten times compared with a so-called GaAs-HEMT. Therefore, GaN-HEMTs are expected as electronic devices having withstand voltages largely exceeding those of GaAs-HEMTs. At the present time, it has been reported that GaN-HEMTs can exhibit withstand voltages higher than 200V under the current-off state.
FIG. 11 illustrates an example of the configuration of a conventional GaN-HEMT.
In the GaN-HEMT, an AlN layer 102 and a GaN layer 103 are laminated in order, for example, on a SiC substrate 101 to form a buffer layer. Further, an n-type AlGaN layer 105 is laminated on the GaN layer 103. A gate electrode 106 which is Schottky-connected to the AlGaN layer 105, and a source electrode 107 and a drain electrode 108 which are spaced apart from the both sides of the gate electrode 106 on the AlGaN layer 105 and are ohmic-connected thereto are formed by pattern formation. Further, a SiN layer 9 as a protective insulating film is laminated between the source electrode 107 and the drain electrode 108 on the AlGaN layer 105. Thus, a GaN-HEMT is configured.
However, in the case of using a GaN-HEMT as an electronic device having high withstand voltage, there is a problem that the characteristics thereof vary largely. For example, FIG. 12 illustrates the relationship between the time and the output value when a conventional GaN-HEMT is continuously used.
Thus, as a GaN-HEMT is used a number of times, the output value thereof is decreased to about 70% of the initial value after continuous use thereof, for example, for 15 years. The reduction of the output value by about 30% after a lapse of 15 years can not be said to be sufficiently practical, and thus conventional GaN-HEMTs can be considered to have a significant problem in this regard.